Process of hydrating calcined gypsum



Patented Feb. 18, 1941 UNITED STATES PROCESS OF IEYYPPRATING CALCINED Joseph W.

States Gypsum Company,

poration of Illinois No Drawing. Original Serial No. 666,352. tion October 24, 1936,

1 Claim.

. the coating material. Satin white is made by treating an alum solution with lime so that the resulting product contains a substantial proportion of aluminous gel, which gives the coating material certain characteristics that are objec- 15 tionable. The use of pure recrystallized and deflocculated ypsum, free of aluminous gel, instead of satin white, gives a coating which, although acquiring a scmewhat'lower (less glossy) finish on calendering than satin white coatings, will show excellent levelness or smoothness, which is necessary to permit high grade printing. The coating which can be produced from my special form of gypsum also takes ink exceptionally well and in addition has other importtant advantages. A coating comprising the improved defiocculated gypsum base shows less tendency to darken when heavily calendered than does a coating material with a satin whitebase. The deflocculated gypsum requires less casein to secure a given adhesion of the coating to the body of the paper than that required when satin white is used as a base pigment. The deflocculated gypsum coating material can be dried, disintegrated, and then readily mixed again with with satin white finishes, which are not readily re-dispersed properly after drying, owing to the presence of partially dehydrated aluminum oxide which is not reversible to gel form. I r

In hydrating plaster of Paris to form deflocculated hydrated gypsum, thought necessary to use a relatively large amount of water (from part plaster by weight) to prevent the slurry from setting during the agitation when a paddle agitator is used. The use of this large amount of water at a paper coating mill would be a considerable inconvenience, as it necessitates a partial dewatering of the prepared recrystallized gypsum slurry before it can be mixed with casein glue to form a color of consistency suitable for coating paper. This point is of particular importance as many coating mills do not have dewatering equipment such as filter presses and centrifuges which would be water to form a smooth slurry, which is not true it has heretofore been 4 to 8 parts water per I SUM Gill, Elmhurst, 111.,assignor to United Chicago, 111., a corapplication April 15, 1 933,

Divided and 3 this applica- Serial No. 107,371 (Cl. 134"-58) necessary to remove the excess water. However, while I have found it advantageous to prepare the paper coating with a much smaller amoun'tlof water than above indicated, yet, if

desired, thev deflocculated gypsum may be pre- 5 pared by the use of a large amount of water to produce substantially the same end product; for I believe the use of recrystallized and/0r deflocculated ypsum as a paper coating material to be new regardless of its method of manufacture.

I'have found that by adding certain electrolytes in'srnall amounts to the water in which the plaster is hydrated to. form deflocculated gypsum, the tendency for the slurry to stiffen ex- 7 cessively or set during the hydration while be-' ingflstirred in any suitable agitator, is consid- 'erablyv reduced, thus permitting the manufacture "of defiocculated gypsum from a mixture of 1 2 to Zparts water per part of plaster by weight.,20 A deflocculated gypsum so formed can be compounded to a satisfactory coating color by merely adding the requisite quantity of casein glue. I have found that from 1 /2 to 2 parts by weight of an alkali phthalate per 100 parts of calcined. gypsum plaster, are effective in reducing the setting tendency of the plaster, although these limits can be widened considerably without essentially changing the characteristics desired in the process. A still more effective electrolyte is sodium phthalata'especially when made alkaline with a slight excess of caustic soda. The potassium salts of the above named electrolytes may also be-used, if desired.

The exact reason for this action of these elec- ..trolytes is not completely known. Their effect onthe size and shape of the gypsum crystals obtained on hydrating the calcined gypsum plaster will, undoubtedly, aliord a partial explanationpof the action. The crystals of hydrated gypsum, particularly when the alkaline-phthalate is used, have a smaller length to breadth ratio than when ordinary plaster-water mix or plaster-water mix containing small amounts of other salts are used.

In general, as the particle size decreases, the consistency, the casein required for a coating material, the opacity or hiding power, and hence, the whiteness of the applied coating, and the levelness or printing smoothness of the coating, increase. The glare obtainable seems to vary but little with change in particle size, but can be varied considerably by varying the calendering process. An additional advantage of the controlled shape and size obtainable with my improved. gypsum pigment lies in the improved spreading qualities obtainable, for the spreading characteristics of precipitated gypsum color depends somewhat on the ratio of crystal length to thickness, an excessive length generally causing poorer spreading characteristics.

The product resulting when the calcined gypsum is agitated with the above amount of water, containing the electrolyte, consists of a slurry having about the consistency of stifi cream. This fluid slurry, after about one hour of agitation, contains defiocculated, completely hydrated gypsum in recrystallized form, the crystals being plate-like and about 4 times as long as broad, while gypsum similarly hydrated but in the presence of substantially water alone has needle-like crystals measuring about 13 to 25 times as long as broad. These crystals average about 5.4 microns in length while crystals produced without an electrolyte and with larger quantities of water average about 14 microns in length. A coating composition {suitable for many purposes can be prepared by simply adding the proper amount of adhesive, such as casein glue, to the slurry. However, in many other cases it will be found desirable to incorporate other materials well known to the art, along with the adhesive and my improved gypsum pigment, to secure a coating composition having the properties desired.

As a specific illustration of the application of this invention in the preparation of a caseincontaining coating color, the following formula has been used with satisfaction: 8.5 parts by weight phthalic acid, and 5.5 parts of sodium hydroxide were dissolved in 1300 parts of water at room temperature. 850 parts of white molding plaster or calcined gypsum were addedto this solution and the mix was stirred for one hour in a standard mechanical mixer. The slurry produced was now a viscous fluid. To this slurry were added 1100 parts of casein glue containing 1'70 parts of dry casein. 'It is immaterial whether the sodium phthalate is added as such, or whether it is formed in the solutionby the use of phthalic acid or anhydride, together with the equivalent amount of sodium hydroxide,

a slight excess of the latter being desirable.

The resulting mixture after straining represents the coating color which is used directly on the paper coating machine.

The equipment required for making the de flocculated gypsum, is an agitator of suflicient strength that a positive agitation or kneading will be given to the whole plaster-water mix, together with such accessories as storage tanks,

piping, weighing and measuring devices, etc.

Since plaster, mixed at the consistency required in this process when soda ash is used as the set inhibitor does show a distinct, though reduced,

tendency to set during hydration, the agitator must be suinciently powerful to keep the slurry in fluid motion at all times. The maximum speed that can be used without throwing the material out of the agitator gives the best results. A power driven bakery mixer gives good results.

The defiocculated gypsum coating colors are handled on the coating machine in the usual manner. It may be found advisable to run the machine at a somewhat higher speed than usual, or else use a somewhat harder sized raw stock,

as the paper stock seems to draw the water from the deflocculated gypsum coating faster during the setting stage of the operation than it does from a clay coating. The deflocculated gypsum coatings are relatively more satisfactory in medium and heavy coatings than in light (below 10 pounds) coatings. The essential point in finishing defiocculated gypsum coatings is to calender the paper with decidedly more pressure than is usually used, particularly, more than is used on'satin'whi-te coatings. This can be safely done, as thedefiocculated gypsum coating is substantially free from any tendency to blacken under pressure up to the limit the fiber will stand without-crushing. The-heavy calendering aids in bringing out the levelness characteristic of-deflocculated gypsumcoatings and in increasing the glare, and thus improves the printing qualities-of the stock.

The main advantages of my improved defiocculated gypsum coating are:

I would state in conclusion that while the examples described constitute practical embodi- -ments-of myinvention, I do not wish to limit myself precisely to these details, since manifestly, the same may be considerably varied without departing from the spirit of the invention as defined in the appended claim.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

A process of hydrating calcined gypsum without substantial-setting thereof "which comprises agitating one part of calcined gypsum with substantially 1 to 2 parts of -a dilute aqueous solution of an alkali metal phthalate.

JOSEPH 'W. GILL. 

